There are different types and causes of concrete discoloration. Dark, shadowy areas may appear if the fresh concrete was placed over alternating wet and dry areas of the subgrade. Darker areas are typically located where the subgrade was too wet or actually muddy. Temporarily covering concrete with poly sheeting or construction blankets can also cause the hardened concrete to be darker in appearance. Another possible cause of dark, mottled areas is the uneven application of curing/sealing products. Curing/sealing products should be applied as uniformly as possible with a nozzle that will produce a fine spray or mist. There can also be differences in coloration between concrete slabs placed on different days. This is usually due to slight variations in temperature and weather conditions from day to day. The potential for discoloration can be greatly reduced by following recommended placing, finishing and curing practices.